This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-353143, filed Nov. 20, 2000, the entire contents of which are incorporated herein by reference.
Field of the Invention
The present invention relates to a spindle motor for driving a disk, polygon mirror, etc. More specifically, the present invention concerns an attachment technique associated with a configuration of a spindle motor for improving accuracy of a rotor.
In recent years, there are increasing demands for low-profile and low-cost motors for driving disks or polygon mirrors. Furthermore, as the density and speed of disks are increased, the accuracy of the disk-mounting surface must be improved, rotational irregularities must be ironed-out, and vibration noise must be reduced.
The requirements of high rotational speed, high reflective accuracy of the polygon mirrors, and low vibration noise have placed tough demands on the polygon mirror mounting surface. To meet these demands, a sliding bearing is used to reduce costs, whilst a hydrodynamic bearing is used for high accuracy, high speed and cost reduction.
Therefore, the major problems of improving the accuracy, decreasing vibration noise and rotational irregularities in polygon mirror or disk-driving motors need to be overcome, whilst also satisfying the demands of cost-reduction and the motor to be made lower-profile. As a solution to the above problems of accuracy and vibration, Jpn. Pat. Appln. KOKAI publication No. 8-19236 proposes the following:
The flat-type brushless motor described in Jpn. Pat. Appln. KOKAI Publication No. 8-19236 comprises a rotor hub, a rotor yoke, and a field magnet. A rotation shaft is pressed into the motor rotor. The rotor yoke is caulked to the rotor hub. A plurality of positioning protrusions is formed by pressing a rotor yoke""s substrate for mounting a flat surface of the field magnet. The field magnet slides on the substrate and moves between positioning protrusions. After positioning is complete, an outer periphery of the field magnet is fixed by means of adhesive.
A circular wall is formed at an external wall on the substrate of the rotor yoke. Inside this circular wall, an inside diameter is formed by the positioning projection. In the inside diameter, the field magnet""s outside diameter and plane surface directly contact the rotor yoke and are fixed with adhesive.
However, the prior art for motors proposing the aforementioned configuration and mounting technique presents the following problems due to parts and positioning inaccuracies. For example, a part called a rotor yoke is installed between the field magnet and a rotating shaft working as a rotation center of the field magnet. The sum total of the inaccuracies inevitably increases the eccentricity. Generally, a field magnet is fabricated by sintering magnetic powder. Due to this, field magnets are fragile and break easily. When the field magnet is assembled by sliding on the substrate until positioned at positioning protrusions of the rotor yoke, the field magnet is positioned to one side of a gap between the magnet and the positioning protrusion.
Generally, a rotor yoke is formed by pressing. If a specified length or more is ensured between the protrusion and the circular wall, a plastic deformation ratio of the material used increases. It is difficult to precisely form the circular wall and protrusion. In order to ensure a certain accuracy, an outside diameter of the yoke needs to be substantially larger than the field magnet diameter. As a result, the yoke becomes considerably heavy. Further, if eccentricity exists, the rotor balance is greatly affected in addition to the problem of increased weight. The increased vibration noise associated with the above creates a further problem.
Generally, caulkable metal is used as a material for a rotor hub caulking the rotor yoke. It is common practice to cut a rotating shaft by chucking in order to form a rotor hub""s upper surface with high precision for mounting a polygon mirror, etc. Since a metal is used, machinability is bad, making it difficult to provide the desired precision. Since the mounting surface is thick in the shaft direction, the specific gravity is high and the rotor is heavy. Further, vibration noise increases due to the rotor""s eccentricity.
Especially, the length of the rotation shaft and portion pressed into the hub need to be reduced to enable a low-profile motor. However, this reduces the rigidity of the pressed portion. When a polygon mirror mounting surface of the rotor hub is cut, the use of a metal having a large cutting resistance makes it difficult to ensure the desired precision. Consequently, this degrades the accuracy of the polygon mirror""s reflective surface.
The field magnet and the rotor yoke are fixed with adhesive after positioning, degrading the workability and increasing costs. The whole of one plane and the outside periphery of the field magnet are directly fixed to the rotor yoke. The rotating shaft is pressed by caulking the rotor yoke and the hub, and is fixed to the base plate via bearings. In this configuration, vibrations caused by the drive torque occur between a stator and the field magnet. This vibration propagates from the field magnet to the base plate, and increases. The rotor yoke, hub, and rotation shaft of the rotor section are made of metal. The field magnet is also made of a semi-ferrous material. This easily causes resonant frequencies approximate to each other.
During operation, there is a possibility that the apparatus housing the above-mentioned drive motor will be dropped. This motor may be provided with bearings such as a sliding bearing or a hydraulic bearing for supporting a shaft, using a thrust pad. In such a case, there is a high possibility that the shock due to the drop causes the rotor to be dismounted. It is therefore necessary to provide low-cost means for preventing disengagement.
As mentioned above, many problems are pointed out as far as the prior art is concerned. The positioning accuracy of the motor components of the prior art is still unsatisfactory. Accordingly, the above problems impose limitations on effective solutions for vibration and noise.
It is therefore an object of the present invention to provide a spindle motor mountable with increased accuracy, together with a field magnet thereof.
In order to solve the above-mentioned problems and achieve the object, the present invention devises the following means. According to one mode, a spindle motor includes a base plate, a stator mounted on the base plate, a bearing provided on the base plate, a rotating shaft rotatably provided on the bearing, and a rotor fixed to the rotating shaft. The rotor is formed of a metallic magnetic substance and resin and is fixed to the rotating shaft at the magnetic substance, and a field magnet fixed to the rotor. The field magnet is pressed into and fixed to the rotor at a resin section of the rotor.
The bearing is a dynamic pressure bearing, including a sleeve and a thrust pad.
The stator includes an auxiliary yoke, and the rotor includes a mounting surface for mounting a disk or a polygon mirror at a resin section.
The rotor includes a rib formed of the resin, and the field magnet touches the rib and forms a gap with a specified distance.
The resin section is placed between the magnetic substance and the field magnet, and the resin section is provided with the rotor forming ribs so that a specified gap is provided between a plane of the field magnet and the resin section.
The present invention aims at decreasing the resonance and vibration noise by using the very low vibration transmissibility of resin.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.